A fox ate two of the Tower of London's eight prized ravens donated to mark the Queen’s Diamond Jubilee - and came close to threatening the monarchy.

The double killing sparked an inquiry after the royal birds - which have mythical status due to a 350-year-old legend - met a grisly end in the grounds.

The creatures are carefully looked after due to King Charles II decreeing there must always be at least six ravens in the tower or the kingdom and the monarchy would fall.

One of the birds, called Jubilee, was accepted by The Princes Royal on behalf of The Queen in March 2012.

Having spent six months in captivity acclimatising to castle life he was released in December with much fanfare alongside a feathered companion called Gripp - named after Charles Dicken’s pet raven.

The pair were a gift from Martin Harris, a Somerset breeder who has been supplying the Tower for 13 years.

But despite the creatures being regarded as the most pampered and intelligent birds in the country, a cunning urban fox managed to swoop on the ravens shortly before they were due to be locked up in their cages for the night. They were unable to fly to safety as their wings are clipped.

A spokeswoman for Historic Royal Palaces, an independent charity which runs the Tower, said: “Raven deaths at the Tower are not common, with many ravens living long healthy lives here.

“In recent years there have been no fatalities due to foxes, a natural raven predator, and the ravens are put to bed each evening in secure enclosures to minimise these risks. However sadly in May 2013 a fox captured two of the ravens before they were put to bed.

“We take the welfare of the ravens seriously and have been reviewing our current pest control measures.”

The Tower spends £4,000 a year on the birds’ upkeep and is preparing to upgrade their cages for the first time in nearly 20 years.

The change is part of a project to ‘modernise’ the tourist attraction and bring visitors closer to the birds.